Microwave tube



Dec. l, 1959 R. H. BARTRAM 2,915,666V

MICROWAVE TUBE RAL PH H. BA RTRAM Mmm A TTORNE Y 2,915,666 t Patented. Dec. .1, 1959 IVIICROWAVE TUBE Ralph H. Bartram, Kew Gardens, N.Y., assignor, by mesne assignments, to Sylvania Electric Products Inc.,

Wilmington, Del., a corporation of Delaware Application August 28, 1957, Serial No. 680,761

3 Claims. (Cl. 313-156) My invention is directed toward strophotron oscillators.

A strophotron oscillator is a multitransit electron tube adaptable for use in the VHF and UHF frequency ranges and is described for example in an article entitled A New Electron Tube: The strophotron by Hannes Alfven and Dag Romell, and published in the Proceedings of the`I.R.E., page 1239, Vol. 42, No. 8, August 1954.

The strophotron described in this article includes an elongatedaccelerator, first and second separated plane reliectors, a cathode and a collector. The accelerator extends in a given direction and has a uniform cross section, a selected portion of the accelerator surface having a predetermined curvature. The two reiiectors extend in the same direction and are symmetrically disposed about the curved portion of the accelerator surface. The refiectors are not parallel to each other, however, and are so positioned that the reflector separation is maximum in a region adjacent the curved portion of the accelerator surface. The cathodev is positioned in a slot in one reflector adjacent one end of the accelerator and the collector is positioned adjacent the other end of the accelerator. A load, for example, a resistive load is coupled between the two reflectors.

The accelerator is maintained at a high positive potential relative to the refiectors, this establishing an electric field therebetween. Since the reflectors are not parallel and a selected portion' of the accelerator surface adjacent the refiectors is curved, the electric field is non-uniform.

A uniform magnetic field is established between the accelerator and the refiectors, the magnetic field vector pointing in a direction perpendicular to a plane of symmetry extending through the accelerator and between the refiectors.

Under these conditions, certain of the electrons emitted from the cathode will migrate toward the collector along a curved path. The projection of this path onto the plane of symmetry resembles a trochoid, while the projection of the path onto a second plane perpendicular to the plane of symmetry resembles a damped sinusoid varying about an axis defined by the intersection of the two planes.

The sinusoid or oscillator frequency is determined by the intensity of the electric field, while the trochoid frequency is determined by the intensity of both the electric and magnetic fields.

Due to the geometry of the device, the electric potential at any point within an essentially planar region perpendicular to the plane of symmetry and adjacent the curved portion of the accelerator surface is proportional to the negative of the square of the distance between this point and the plane of symmetry.

The electrons oscillate in the second plane at the sinusoid frequency. As the electrons swing back and forth between the reflectors, energy is delivered to the load and the electron oscillations are damped. Ultimate- 1y, the electrons impinge on the collector and are removed from the electron interaction zone of the device.

As long as the electrons are confined within the planar region, the oscillation frequency remains fixed.

However, in actual operation of the device, since the electron beam has a finite cross section and the trochoidal motion has a vertical componentthe velectrons are not confined to this single planar region, but rather will also occupy other adjacent regions. (The magnetic field intensity is adjusted to prevent any electron from striking the accelerator.) Under these conditions, since the electric potential distribution outside of the single planar region differs from that inside the region, the sinusoid or oscillator frequency will not remain constant as electrons move outside of the selectedl region, but instead will vary depending upon the separation between the l electrons and the planar region.

I have invented a new type of strophotron which overcomes this difiiculty. Y n

Accordingly, it is an object of the present invention to prevent the generation of undesired frequencies in a strophotron.

Another object is to eliminate oscillator frequency variation in a strophotron.

Still another object is to provide a new and improved strophotron in which the oscillator frequency is held constant and undesired frequencies are not produc'ed.

In accordance with the principles of my invention, I provide an elongated accelerator having a uniform cross section defining an hyperbola branch as, for example, an upper branch of an hyperbola. First and second separated plane refiectors are so positioned as to coincide with corresponding portions of the first and second asymptotes of the upper hyperbola branch.

The accelerator is maintained at a high positive potential relative to the refiectors, thus establishing lan electric field therebetween. his field, due to the geometry of the accelerator and reflectors, creates an hyperbolic electric potential distribution within the entire region bounded by the accelerator and the refiectors. Stated differently, the potential'V of any point within this region is proportional to the quantity (y2-z2), wherein y is the vertical separation between this point and the intersection of the first and second asymptotes, and z is the horizontal separation between the point and the central point of the upper hyperbola brauch.

A uniform magnetic field is established within this region, the magnetic field vector pointing in a direction perpendicular to the plane of symmetry.

A cathode Ais mounted in one of the reflectors adjacentl one end of the accelerator and an anode or collector maintained at a high positive potential relative to the reiiectors is positioned adjacent the other end of the accelerator. A load which can be, for example, a resistor or a resonant circuit tuned to the oscillator frequency is coupled between the two reflectors.

Under these conditions, electrons emitted from the cathode will again migrate toward the collector along a curved path and will exhibit the characteristic strophotron behavior; i.e. the projection of this path onto a plane of symmetry extending through the axis of the accelerator and between therefiectors resembles a trochoid while the: projection of this path onto a second plane perpendicular to the plane of symmetry resembles a damped sinusoid having an axis defined by the intersection of the tw@ planes.

in contradistinction to the prior art, however, the fre remains constant.

An illustrative embodiment of my invention will now be described with reference to the accompanying drawings, wherein Fig. l is an isometric view of a strophotron in accordance with my invention;

Fig. 2 is a cross sectional view of the strophotron of Fig. l and also illustrates the electric potential distribution between the accelerator and the planar reflectors shown in Fig. l; and

Figs. 3a and 3b are graphs of the motion of favorably phased electrons in the interaction region between the accelerator and the reflectors.

Referring now to Fig. l, enclosed in an evacuated tube envelope (not shown) is an elongated accelerator l0. For ease of illustration, the envelope is not shown in Fig. l but is shown in cross section and identified at 19 in Fig. 2. Accelerator 10 has a uniform cross section defining an upper branch of an hyperbola having a central point l5. First and' second separated plane reliectors 16 and 18 are positioned to substantially coincide with corresponding portions of the left hand and right hand asymptotes defined by the upper branch of the hyperbola. The reflectors and the accelerator extend along the same direction.

By virtue of this arrangement, a plane of symmetry xy(z=) extends through the central point l5 equidistantly between the reliectors 16 and 18. A second plane xz(y=0) is perpendicular to plane xy.

A uniform magnetic field is established within a region containing accelerator lt) and reflectors 16 and 18, the magnetic field vector H pointing in the -z direction. (Means for establishing this field are conventional and are not shown here.)

Accelerator l() is maintained at a high positive direct potential relative to both reflectors lo and 18. Due to the geometry of the accelerator and the reflectors, the resultant electric field creates an hyperbolic electric potential distribution within the entire region bounded by the electrode and the reflectors. (Note that the hyperbolic portion of the accelerator and the reflectors are both extended sufficiently to substantially eliminate fringe electric fields.)

A cathode 22 is positioned in a slot 24 in reflector 16 at a point adjacent one end thereof, and a collector 26 (maintained for convenience at the same potential as accelerator lll) is positioned adjacent the other end of reflector 16.

A load 28 which can be purely resistive but, in this example, is a resonant circuit tuned to the oscillation frequency, is coupled between reflectors 16 and 18.

Electrons are emitted from the cathode at an extremely low velocity and enter the interaction region. Many of these electrons then drift toward the collector along a curved path and exhibit the characteristic strophotron behavior.

More particularly, the projection of this path onto the plane of symmetry xy (shown in Fig. l) resembles a trochoid as shown in Fig. 3a. Further, the projection of this path onto the second plane xz (shown in Fig. l) resembles a damped sinusoid as shown in Fig. 3b. (For the purposes of clarity the z component of trochoidal motion which is superimposed upon the sinusoid has not been shown in Fig. 3b.) The sinusoidal frequency is the oscillation frequency and is primarily determined by the electric field established between the accelerator and the plane reflectors; the trochoid frequency is determined by both the magnetic field and the electric field and is independent of the oscillation frequency. (The magnetic field intensity is adjusted to prevent electrons from irnpinging on and being collected by the accelerator.)

The hyperbolic electric field distribution between the accelerator and the reflectors is plotted graphically in Fig. 2. As will be seen from Fig. 2, the electric potential V for any point between the accelerator and the reflector electrode is proportional to the quantity (y2z2) where l y is the vertical separation between this point and the intersection 50 of asymptotes 52 and 54, and z is the horizontal separation between this point and the central point 15 of upper hyperbolic branch 14, The equipoteu tial surfaces 56 are families of right hyperbolas with the lines yiz as asymptotes.

The oscillation frequency, as indicated previously, is determined by the electric field. Further, the electrons, in moving toward and away from the reflectors, induce an alternating voltage of oscillator frequency across the load, and this voltage acts on the electrons to produce the damping action. More specifically, when an electron leaves the cathode and enters the interaction region at such times as to be in phase with the induced voltage (such an electron is termed a favorably phased electron), it will oscillate back and forth between the plates in the manner indicated. The electrons which enter the interaction region at such times as to be out of phase with the induced voltage impinge on a reflector on the first or second pass, and are thus removed from the interaction region. Hence, only the favorably phased electrons remain in the interaction region for an appreciable period, and the oscillator voltage appearing across the load is not substantially affected by out of phase electrons.

The initial amplitude of the sinusoidal motion of any electron is determined by the vertical displacement between this electron and the central point of the hyperbola branch, the amplitude decreasing as the displacement in- ClSaSeS.

However, by virtue of the hyperbolic electric field, the oscillation frequency is independent of vertical displacement.

While I have shown and pointed out my invention as applied above, it will be apparent to those skilled in the art, that many modifications can be made within the scope and sphere of my invention as defined in the claims which follow.

What is claimed is:

l. A strophotron comprising, within an evacuated tube envelope, an elongated accelerator positioned within said envelope and extending in a given direction therein, said accelerator having a uniform cross section defining an upper branch of an hyperbola; first and second separate plane reflectors respectively positioned along corresponding portions of the left hand and right hand asymptotes of said hyperbola branch, said reflectors extending in said direction; means to maintain said accelerator at a high direct potential relative to said reflectors to establish an hyperbolic electric potential distribution within a region bounded by said accelerator and said retlectors, the potential V at any point positioned within said region being proportional to the quantity (y2-z2) wherein y is the perpendicular separation between said point and the point of intersection of said asymptotes and z is the perpendicular separation between said point and the central point of said hyperbola branch.

2. A strophotron having within an evacuated tube envelope an elongated accelerator having a uniform cross section defining an upper branch of an hyperbola; first and second separate plane reflectors respectively positioned along corresponding' portions of the left hand and right hand asymptotes of said hyperbola branch, said reflectors extending in said direction; said first reflector being provided with a slot adjacent one end of said accelerator; a cathode mounted within said slot; a collector positioned adjacent the other end of said accelerator; means to maintain said collector and said accelerator at high direct potentials relative to said reflectors to estab lish an hyperbolic electric potential distribution within a region bounded by said accelerator and said reflectors, the potential V at any point positioned within said region being proportional to the quantity (y2-z2) wherein y is the perpendicular separation between said point and the point of intersection of said asymptotes and z is the perpendicular separation between said point and the central point of said hyperbola branch; and means to establish a uniform magnetic field within the region bounded by said accelerator, said reflectors and said collector, said accelerator and said reflectors defining a plane of symmetry extending equidistantly between the reectors and through the central point of said branch in said same direction, the magnetic iield vector pointing in a direction perpendicular to said plane.

3. A strophotron comprising, within an evacuated tube envelope, an elongated accelerator positioned within said envelope and extending in a given direction therein, said accelerator having a uniform cross section dening an upper branch of an hyperbola; rst and second separate plane reflectors respectively positioned along corresponding portions of the left hand and right hand asymptotes of said hyperbola branch, said reilectors extending in said direction; means to maintain said accelerator at a high direct potential relative to said reflectors to establish an hyperbolic electric potential distribution within a region bounded by said accelerator and said reectors, the potential V at any point positioned within said region being proportional to the quantity (y2-z2) wherein y is the perpendicular separation between said References Cited in the iile of this patent UNlTED STATES PATENTS 2,161,466 Henneberg June 6, 1939 2,206,668 Hollmann July 2, 1940 2,413,251 Smith Dec. 24, 1946 2,414,121 Pierce Jan. 14, 1947 2,745,039 Bowen May 8, 1956 2,834,908 Kompfner May 13, 1958 FOREIGN PATENTS 1,074,543 France Apr. 7, 1954 729,930 Great Britain May 11, 1955 

